1. Field of the Invention
The present invention relates generally to electronic packaging of semiconductor integrated circuits, and more particularly, to electronic packaging of photo-sensing semiconductor devices.
2. Prior Art
Photo-sensing semiconductor devices have been typically mounted in ceramic packages. FIG. 1 shows a schematic cross-section view of a ceramic leadless chip carrier (CLCC), which is the most popular package format for photo-sensing devices. As shown in the figure, a photo sensing semiconductor die is mounted face-up on a ceramic substrate 4 by using epoxy or the like inside of an enclosure that is covered by a glass lid 6. Wire bonding 8 is typically used to connect the photo-sensing die 2 to the ceramic substrate 4. Solderable pads 10 are provided on the bottom of ceramic substrate 4 to connect the package to a circuit board.
Perhaps the most critical drawback of this package is that it is very expensive. Another drawback is that the package size is not small enough for some hand-held applications such as cameras in cellular phones where small size and light weight are essential features. Another drawback is that the package's configuration does not allow very precise placement of the photo-sensing device in relation, for instance, to the focal plane of a lens because, among other things, the photo-sensing die is mounted with epoxy or the like, and the package itself is mounted with solder paste.
U.S. Pat. No. 5,302,718, entitled “Semiconductor Insulation for Optical Devices,” teaches the mounting of a photo-sensor on a printed-circuit board by integrating a sensor, a lens, and a molded mount in a molded mount package providing locating pins. The aforementioned Patent provides limited improvements in positioning accuracy for the sensor with respect to its primitive lens system. It also provides at best mediocre accuracy in the placement of the package itself on a mounting board.
Another known packaging approach for photo-sensing semiconductor devices is one offered by Shellcase, Inc. Detailed techniques are disclosed by U.S. Pat. Nos. 5,716,759, 6,040,235, and 6,117,707. FIG. 2 shows a schematic cross-section of a package formed in accordance with those techniques. A patterned metal layer is applied to a photo-sensing semiconductor wafer to extend bonding pads to its dicing area having a narrow width between neighboring dice. A photo-sensing wafer is attached to a glass substrate by using epoxy. After that, the backside of the wafer is ground for to thin out the wafer. The silicon of the dicing area is then removed to expose metal lines. Many more process steps are needed to complete fabrication, but a detailed explanation is omitted, as such is not necessary for a clear understanding in the present invention.
The advantage of this package compared to a CLCC package is its smaller size. A number of drawbacks are nonetheless found in this package as well. Perhaps the most critical drawback with this package is the complexity both in structure and fabrication process. This complexity is a significant factor in mass production since complexity tends to increase the processing yield loss. As a result of its complexity and attendant yield loss, the package is expensive to fabricate.
Other nontrivial disadvantages of this technique include its need for a wide dicing line, which cuts against the trend in semiconductor manufacturing to decrease the width of dicing lines to achieve more dice per wafer. The current typical dicing line width of about 100 micrometers is not wide enough to support this technique. Consequently, the packaging technique is not compatible with semiconductor wafers having standard dicing line widths, and requires customizing measures to ensure the wider than usual dicing line widths.
Accordingly, it is an object of the present invention to provide a lower cost package for photo-sensing devices. Another object of the present invention is to provide a sufficiently compact package for photo-sensing devices to accommodate hand-held applications like cellular phone cameras where the small size is possibly the single most important packaging factor. Yet another object of the present invention is to provide a simple and conveniently fabricated and assembled package in which precise positioning of a focal plane in the horizontal as well as the vertical plane is effected in an effortless manner.